A new idea has made me torn

I've been working on a huge fantasy novel for two and a half years, literally being my first child when it comes to writing. I have every plan to attempt to get this published once finished, which I assume will take a few years more.

But now...a new idea struck me, an opening line that hooked me immediately, and opened a new universe to my brain to dwell in. At first I thought this could be large scaled book as well, and it could, but I think it would stand better as a short story, perhaps with some larger scale work later. Now this genre sets me into uncharted territory. Science fiction. The reason for not wanting to make this a larger scale work means a lot more research, probably having to hit up with professors and think about real-life hypotheticals. Anyways I digress...

I feel like this new idea is trying to take over my first born, my dear dear fantasy novel. Since I decided the sciene fiction story to be a short story, it would be a maximum of 7500 words, right? And I've been wondering...do I press pause on this fantasy novel that might end up at 300k++ words, and write this new sci-fi story? Or do I finish what I started?

I am afraid, I know it's highly unlikely, but who knows, what if I go over to that science fiction story, and fall completely out of tune with the fantasy one? I really want to press on with the fantasy one, but right now I'm completely mesmerized by the sci-fi story, and I think with this jolt of inspiration I might get it done quickly...I DON'T KNOW WHAT TO DOOOO. I CAN'T JUST LEAVE MY BABY! AND NOW I'M RANTING HELP I NEED CPR.

edit: I'm highly intoxicated on the drug called "missing sleep" and I just feel like I fucked up my title...has is right, right? haa...and I'm supposed to write movels. I think some sleep will do the trick

Many writers switch back and forth between works whenever they have inspiration for one or the other. There's no reason why you can't do that too. There's no rule that says you can only work on one thing at a time lol. If you're afraid of losing touch with the fantasy novel, then give yourself some structure and some discipline and just make sure you work on both during the week, or every other week, whatever works, rather than necessarily sticking to what you feel like all of the time. (key word being "all" - there's nothing wrong with doing what you feel like but if the aim is to keep both stories up at the same time, then some enforced structure might be needed)

Other writers prefer to focus on one story at a time. In this sense, you could jolt down the sci-fi idea and shelf it for later, when you hit a block with your fantasy and need something new to work on temporarily.

In any case, both stories will still be there for you to work on whenever you choose to work on it. There's no need to see it as like one is "taking over" the other. Go with the flow.

Btw at 300k++ words, you'll need to likely reduce the word count or split it into multiple books at the end anyway, so the fantasy probably won't be finished anytime soon. In that respect, it's probably good to have a side project to hop into to freshen things up from time to time Working on the same project for too long can squeeze you dry in the long run.

I agree with @Mckk . There's no ground rule carved in stone that says, “Thou shalt work on one, and only one, story at a time!! So is it written!!” You can work on as many stories as you want, but just be mindful. It's entirely possible to get so many projects piled onto you that you just freeze. Pick a few stories that inspire you and get to work on them. Make a schedule such as one week per project so they all flow at basically the same rate. Have fun!

I agree with everyone else. You can switch it up, especially if the other one is a short story. I don't think a short will necessarily derail you from your larger work.

I have been working on my one novel idea for a while now, and I frequently write short stories in-between when the inspiration hits me. Although I write the first draft of my shorts all in the same day, so it really doesn't take too much time from my bigger work. Very recently I had a new idea for a novel, and I decided to shelf that idea because I was afraid starting that would take away focus from my WIP. Of course, it's still there, dancing in the back of my head, which is a little distracting...